Er-doped fiber amplifiers (EDFAs) are widely used in optical fiber communication systems. Traditionally, EDFAs are used in the wavelength range from about 1525 nm to about 1565 nm, a range that is commonly referred to as the C-band (when "C" stands for "conventional").
A significant fraction of installed optical fiber (e.g., dispersion-shifted fiber) is not suitable for multichannel dense wavelength division multiplexed (DWDM) operation across the entire C-band wavelength range because of nonlinear effects such as four-wave mixing. Thus, it would be advantageous to have available an optical fiber amplifier that can operate in a wavelength regime that includes the operating wavelength of the above referred to installed optical fiber, typically greater than 1565 nm.
Recently use of EDFAs in the approximate range 1565-1625 nm has been reported. See, for instance, J. F. Massicott et al., Electronics Letters, Vol. 28, pp. 1924-1925 (1992). This range is now commonly called the L-band, where "L" stands for "long". Operation in both the C-band and the L-band in a single EDFA has also been reported. See Y. Sun et al., Proceedings of the European Conference on Optical Communication (ECOC '98), pp. 53-54 (1998).
To date, the reported performance of L-band EDFAs has been inferior to that routinely available in C-band EDFAs. For instance, the noise figure (NF) of L-band EDFAs has generally been 1-2 dB higher than for C-band EDFAs, and the output power of the former has typically been 1 to several dBs lower than that of the latter. In view of the potential advantages of a L-band EDFA, it would be highly desirable to have available a L-band EDFA with improved performance, desirably comparable to that of C-band EDFAs. This application discloses such an EDFA.
FIG. 1 shows the base modeling parameters for a conventional Er-doped fiber (EDF), with numerals 10 and 11 referring to absorption and gain, respectively. The C-band and L-band are also indicated. The fiber has a numerical aperture (NA) of 0.23, a cut-off wavelength of 850 nm, an Er concentration estimated at 9.0.times.10.sup.25 ions/m.sup.3 in the core, 12M % Al in the core, and adequate Ge to provide the NA of 0.23. The absorption curve of FIG. 1 was measured with all erbium ions in the ground state, and the gain curve was measured in the presence of a high (&gt;300 mW) 980 nm pump power level, which is expected to place all the Er-ions in the excited state (full inversion).
FIG. 2 shows the possible net gain spectra achieved by the fiber of FIG. 1. These are all linear combinations of the gain and loss spectra of FIG. 1, determined according to the formula EQU G(.lambda.)/L=g*(.lambda.)Inv-.alpha.(.lambda.)[1-Inv],
where g* and .alpha. are the gain and loss parameters of FIG. 1, and Inv is the average inversion of the ions along the length of the fiber.
It is known that, in order to achieve a low noise figure (NF) in an EDFA, substantially all ions must be in the excited state. This produces the 100% inversion gain of FIG. 2. In practice, achieving this high inversion gain shape over a substantial portion of the input end of an EDFA (exemplarily enough to produce more than about 10 dB of gain) is adequate to provide a low NF.
It will be noticed that the gain shape of the 100% inversion curve of FIG. 2 is substantially higher in the C-band than in the L-band. Thus, if a highly inverted EDF is long enough to accumulate 10 dB or more of gain across the entire L-band, some wavelengths in the C-band will experience very large gain (exemplarily 50 dB, even 100 dB). In practice, such gain levels are not achieved, for the below-stated reasons. We have analyzed this situation and have found that amplified spontaneous emission (ASE) in the C-band accumulates to a high power level and drives a substantial fraction of the Er ions into the ground state, thereby reducing the gain and increasing the NF. Furthermore, power accumulated and emitted as ASE in the C-band is lost and is not available for emission as signal power in the L-band, thereby decreasing amplifiers efficiency.
Based on our analysis, we have concluded that C-band ASE is typically the largest source of degradation in an L-band amplifier.
EDFAs that operate in at least a portion of the L-band are know, as are EDFAs with an ASE filter. For instance, M. Yamada et al., Electronics Letters, Vol. 33(8), p. 710 (1997) disclose a broad band and gain-flattened amplifier that comprises a 1.55 .mu.m band EDFA and a 1.58 .mu.m band EDFA in parallel configuration. H. Ono et al., IEEE Photonics Technology Letters, Vol. 9(5), p. 596 (1997) disclose a gain-flattened EDFA for use in the 1.57-1.60 .mu.m wavelength region. See also Y. Sun et al., Electronics Letters, Vol. 33, p. 1965 (1997). European patent application No. 94115479.1 inter alia discloses a 2-stage fiber amplifier with an ASE filter between the stages. M. Kakui et al., OFC '96 Technical Digest, WF3, disclose an EDFA that employs a chirped fiber grating as ASE rejection filter for WDM transmission. J. L. Zyskind et al., OFC '94 Technical Digest, WK8, disclose a 2-stage EDFA with counter-pumped first stage that comprises an ASE filter. U.S. Pat. No. 5,406,411 discloses a multistage fiber amplifier including an ASE filter.
U.S. Pat. No. 5,430,572 inter alia discloses ASE filtering for the peak around 1530 nm to help the gain near 1550 nm in the C-band. Such filters are low magnitude narrow band devices. U.S. Pat. No. 5,701,194 discloses differential attenuation of ASE relative to a signal in a "contiguous band". It also shows low magnitude ASE filtering near 1530 nm to help C-band gain near 1550 nm.